This invention relates to establishing a seal between two substrates, particularly where at least one of the substrates is glass.
Many applications exist where it is necessary to secure a glass substrate within a frame such as a metal, plastic or wood frame, which may be painted. For example, glass windshields are secured within the metal or plastic frame of a motor vehicle both during vehicle manufacture and following manufacture to replace the windshield in the event that it cracks or breaks.
It is difficult to establish a strong bond to glass using conventional sealants and adhesives such as polyurethane pastes. To enhance adhesion, the glass surface is typically primed prior to inserting it into the frame.
Polyurethane pastes are conventionally used to establish a seal between the primed glass and the frame. Such pastes, however, are difficult to apply uniformly and reproducibly. Another problem is that pushing the glass into the frame causes the paste to flow and squeeze out of the bond line. This creates bond lines of uneven thickness and glass-frame contact points that can act as failure points because any stress applied to the frame is transmitted directly to the glass at these points. This is particularly a problem when a motor vehicle windshield is installed into a frame that has a highly uneven surface. To address this problem, discontinuous "spacers" are typically placed at various points around the perimeter of the frame. While these spacers help avoid creating glass-frame contact points, they also act as stress concentration points because while the sealant shrinks during cure, the spacers do not. It is then necessary to use extra sealant to accommodate the spacers.
Another problem is encountered in the case of polyurethane sealant pastes that require a relatively long time to cure and build bond strength such as those which are moisture-curable. During this vulnerable curing period, the glass can vibrate within the frame, making the seal and the glass susceptible to damage. Gaps in the seal can form, giving rise to wind noise and compromising seal integrity. The noise associated with the vibrations is also undesirable. Moreover, the reliance on ambient moisture means that the cure process varies depending upon ambient conditions.